Rocket motors 2 using solid propellant are typically comprised of a rocket case 4, usually formed of metal or composite material, a thermal insulation layer 6 lining the interior wall of the rocket casing and the solid propellant 8 (see FIGS. 1 and 2). Running along the longitudinal axis and through the center of the propellant is a central pathway 10. During ignition, the propellant burns and the combustion products pass through the central pathway 10 to the nozzle 11 thereby propelling the rocket.
The rate at which the solid propellant 8 burns establishes the flight characteristics or thrust pattern of that rocket. Unlike liquid fuel rockets, there is no means to control or alter the amount of fuel entering the combustion area during ignition. However, rocket designers have developed a method of forecasting the performance or thrust history of these solid rockets by altering the surface area of the propellant grain exposed to ignition. To achieve this, the propellant grain 8 is designed with specific pathways or slots 11 configured to yield the desired thrust history. In most propellant grain designs the configuration includes a central slot 10 along the longitudinal axis or length of the propellant grain as well as secondary slots 12 formed about the central slot 10. These secondary slots 12 typically extend radially from and coaxially about the central slot 10. However, these slots create areas of stress, particularly at the outer edges 14 of the secondary slots 12. These stresses may produce tears or cracks within the propellant and alter the burn pattern with catastrophic results. It has been found that the amount of stress built up at the outer edge 14 of these secondary slots 12 is a function of the diameter of the secondary slot 12.
One solution to this problem has been to coat the surface of the slot with an organic layer to strengthen the binder in the propellant grain and thereby reduce the impact of the stresses on the propellant. See U.S. Pat. No. 4,052,943.
An alternative approach has been to increase the diameter of the secondary slot 12, thereby increasing the radius of the outer edge 14 and relieving the stress. This approach, although helpful, results in lower propellant loadings, and therefore, less than optimum performance.
Therefore, what is needed in this art is a simple, effective means to relieve the stress at the outer edge of these secondary slots while permitting higher loading efficiencies for the motors.